Heretofore, in some cases, an axial-flow type impeller or a mixed-flow type impeller called inducer is mounted on a distal end of a main shaft to improve suction performance of a pump. Conventional inducer blades have been designed by a design method in which a blade angle along a tip of the blade is designed and a blade angle along a hub is determined based on the tip blade angle by helical conditions. A blade angle from an inlet (leading edge) to an outlet (trailing edge) of the tip of the inducer is designed to be constant or to increase stepwise, linearly, or quadratically in order to meet a head required for the inducer.
In the inducer, it is known that instability phenomena of cavitation behavior called rotating cavitation, cavitation surge, or the like are generated due to the development of cavitation generated on the blades when a pump inlet pressure is lowered. However, in the conventional inducer design method, an inducer blade geometry which suppresses the instability phenomena of cavitation has not been proposed.